Radio frequency identification (RFID) devices are increasingly employed in identification applications. Such RFID applications typically include an RFID device (e.g., RFID-enabled tag, label, etc.) having an identification circuit, a transponder and an antenna that communicate with an RFID reader to identify the RFID device. RFID readers may be deployed at point of sale locations, for instance, to identify goods bearing an RFID device (e.g., tag). In deploying such RFID readers, the location and operating conditions of the readers may vary significantly. Ideally, RFID readers would be placed in electromagnetic-compatible spaces, free of interference from other systems and naturally-induced shielding due to metal parts surrounding the RFID reader antenna and/or the transponder of the RFED device. However, in real-world applications, RFID readers are often installed in environments in which electromagnetic shielding and/or disturbances may occur. When a large conducting body or electric mass is placed in proximity to an RFID reader antenna, it tends affects the electromagnetic or radio characteristics of the typical antenna. For example, an RFID reader may be installed at or near a checkout station, adjacent to one or more electromagnetic shielding or interfering surfaces and/or objects. These types of external bodies tend to cause environmentally induced impedance variations on the RFID reader antenna.
For example, variations of input impedance may be caused by reflected electromagnetic fields. The presence of metallic structures or objects proximate a transmitting antenna tends to cause electromagnetic field scattering, including reflected electromagnetic fields, that contributes to alter the current distribution in the antenna. For instance, the reflected electromagnetic fields may induce additive and/or subtractive currents in the transmitting antenna. Such scattering and/or reflection manifests itself (on the transmitting antenna) as impedance mismatches. Additionally, in some implementations, the transmitting antenna may also be affected by minor background electromagnetic radiation (e.g., shortwave band of 13.56 MHz for an RFID receptor).
In order to counteract these externally induced impedance variations, the RFID reader antenna is typically manually adjusted, at installation for instance, for a particular environment using a separate instrument, such as a Voltage Standing Wave Ratio (VSWR) meter. After initial installation, it may be necessary to readjust the reader, over time, due to the presence of new objects or materials (e.g., shelves, people, or other products) that accumulate near the RFID reader antenna and affect the operation of the RFID reader. Thus, a solution is needed that adjusts the operation of the RFID antenna to approximately maintain a particular antenna impedance.